1. Field of the Invention
The present invention relates to an environmentally benign paint spraying method utilizing a method for environmentally air-drying lacquer dissolved, emulgated or dispersed in water in a paint spraying station. The paint spraying station having a cubicle with a water-rinsed rear wall for collecting and washing out of the overspray in cubicle waste water. The cubicle waste water then being separated by ultrafiltration into permeate and lacquer-containing residue. The permeate being returned to the paint spraying station, and the lacquer-containing residue being enriched with lacquer by removing water until its lacquer content is essentially equal to that of spraying composition which is freshly fed to the spray painting station. The residue that is enriched with lacquer in this way is then admixed to the freshly fed spraying composition for reuse in spray painting. Also admixed therein is a component for preventing coalescence of the lacquer being admixed to the spraying composition fed to the paint spraying station.
2. Discussion of the Prior Art
In prior art devices the overspray usually will be disposed of, which constitutes a heavy burden for the environment and is also very costly. For instance, a binding agent for oven-drying lacquer, which can be dispersed in water, is known from EP-0032554: the disposal of such a binding agent constitutes a heavy burden to the environment, because it contains phosphor. Besides, special measures have to be taken so as to avoid conglutination of the appliances used for separating the overspray from the cubicle waste water, in this context cf. for example DE-3704683.
The separation of cubicle waste water into permeate and lacquer-containing residue by means of ultrafiltration is known, for example, from EP-0307047, EP-0245863, EP-0137877 or EP-0127685. On the other hand, it is known from EP-0318827 or EP-0271015 that the permeate has to be deacidified before it is returned to the paint spraying residue. Both of these aspects of the state of the art are taken into account in EP-0217212.
Experiments for the recuperation of the lacquer from the overspray in cubicle waste water are made reference to in "Oberflache+JOT" 5/1987, page 36, however, in this publication they are considered as "successful only in certain cases". In "Oberflache+JOT" 2/1988, pages 24-25, lacquer and binding agent circuits in the spraying cubicle are qualified as "future", while the recuperation of the lacquer by means of special reprocessing is mentioned as known. Also in DE-3800980, the recuperation of the lacquer by means of reprocessing is known.
In "Oberflache+JOT" 5/1988, pages 61-63, the use of amines for preventing coalescence of air-drying lacquer and the additional cleaning of the cubicle waste water by reverse osmosis following its separation into permeate and lacquer residue are mentioned. In this publication, the recuperation of the lacquer from the overspray is considered as being "in its initial stages of development".
Therefore it is in accordance with prior art that the lacquer-containing residue has to be disposed of.
From EP-0141171 or U.S. Pat. No. 4,607,592 a method for paint spraying with a dispersion of lacquer in water is known. The paint spraying station is equipped with a cubicle with water-rinsed rear wall for collecting and washing out of overspray in cubicle waste water. If the concentration of the overspray in the cubicle waste water reaches about 20% a partial current of the cubicle waste water circuit is conveyed through a filtration unit so as to be separated into filtrate and lacquer-containing residue. The filtrate consists of cleaned water and is returned to the paint spraying station for water rinsing of the cubicle rear wall. At the lacquer-containing residue several parameters are measured such as for example conductibility, in order to regulate filtration in a way that the measured parameters will be approximately equal to those in the freshly fed dispersion. The residue, which is enriched with lacquer in this way, will then be admixed to freshly fed dispersion and thus reused for paint spraying. The overspray in the cubicle waste water is not stable, and it is specified in EP-0141171 or U.S. Pat. No. 4,607,592 that the mixture must pass through dispergers so as to temporarily stabilize it. Besides, it is a drawback of this method according to EP-0141171 or U.S. Pat. No. 4607592 that only a partial current of the cubicle waste water is treated and as a consequence the unit needs a costly pipe and control system. It is probably for this reason that this method failed to gain acceptance (cf. the above quoted article in "Oberflache+JOT").
It is known from JP-49-51324 to enrich the lacquer-containing residue with lacquer by removing water from the total cubicle waste water and to reuse the residue enriched with lacquer in this way for paint spraying. Though it is recommended to use deionized water in order to avoid contamination of the spraying composition, this only serves the purpose of avoiding to soil the spraying composition and it is expressly pointed out that also regular tap water may be used.
Thus, it corresponds to the state of the art that the lacquer-containing residue is not stable and no appropriate countermeasures are known.
As a consequence, the prior art methods of the kind mentioned above are unreliable, which is not acceptable in view of the present demands for the protection of environment.